drm/i915: Decouple execbuf uAPI from internal implementation

At the moment execbuf ring selection is fully coupled to
internal ring ids which is not a good thing on its own.

This dependency is also spread between two source files and
not spelled out at either side which makes it hidden and
fragile.

This patch decouples this dependency by introducing an explicit
translation table of execbuf uAPI to ring id close to the only
call site (i915_gem_do_execbuffer).

This way we are free to change driver internal implementation
details without breaking userspace. All state relating to the
uAPI is now contained in, or next to, i915_gem_do_execbuffer.

As a side benefit, this patch decreases the compiled size
of i915_gem_do_execbuffer.

v2: Extract ring selection into eb_select_ring. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452870770-13981-1-git-send-email-tvrtko.ursulin@linux.intel.com
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 840368d..204661f 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -334,7 +334,7 @@
 		unsigned boosts;
 	} rps;
 
-	struct intel_engine_cs *bsd_ring;
+	unsigned int bsd_ring;
 };
 
 enum intel_dpll_id {
@@ -1299,7 +1299,7 @@
 	bool busy;
 
 	/* the indicator for dispatch video commands on two BSD rings */
-	int bsd_ring_dispatch_index;
+	unsigned int bsd_ring_dispatch_index;
 
 	/** Bit 6 swizzling required for X tiling */
 	uint32_t bit_6_swizzle_x;